CHARACTERISTICS OF MARINE FISHES 167 



swim bladder, except when very young, and it quite surely does not need one. It 

 seems rather significant, however, that the species with primitive lungs, such as 

 the lungfish, fresh-water gars, and tarpon, which do breathe air at least in part, 

 have modified air bladders containing cellular tissue similar to that of the lungs of 

 air-breathing vertebrates. 



Air bladders in some species, as in the drums and croakers, are associated 

 with "voice," the sound of which is little more than a drumming or croaking. This 

 is especially well-developed in the croaker, Micropogon undulatus. It has been 

 reported that during the recent war sonar operators heard a loud noise advancing 

 steadily up Chesapeake Bay and suspected that it was enemy craft. Later it was 

 learned that the noise was caused by a school of migrating croakers. Croaking or 

 drumming is produced by muscles that "beat" on the inflated air bladder. In- 

 cidentally, it is from the walls of the air bladder that isinglass is manufactured. 



The Alimentary Canal. The digestive tract in fishes may be long and coiled, 

 or short and nearly straight. In some species its total length scarcely exceeds the 

 length of the body, while in others it is several times the length of the fish. In 

 general a short simple alimentary canal is associated with fish that feed on an 

 animal diet, and a long complex canal with those that feed on plants. The mud- 

 eating species, such as mullets, Mugil, and gizzard shad, Dorosoma, have 

 stomachs with very heavy muscular walls, resembling the gizzard of a fowl. Most 

 of the cartilaginous fishes have "leaves" in the intestine, forming a structure known 

 as a "spiral valve." Many fishes have blind sacs, called "caeca," attached near 

 the pyloric end of the stomach. These appendices may be few or many, rather 

 long or short. The functions of the different structures of the alimentary canal 

 are not fully understood and cannot be discussed in this brief account further 

 than to remark that each fish is provided with the particular type of digestive 

 system needed for the kind of food it eats. 



Color 



The color of fishes is so variable among species and so changeable within 

 species, or even in the individual fish, that the subject becomes intricate and can 

 be treated only in a very general way. Some species are very somber in color 

 and vary little with age, season, or environment. Others are brilliantly colored and 

 changeable with age, season, and environment. The mosquito fish, Gambusia 

 affinis, is an example of a very modestly colored fish, which at birth already has 

 essentially the color of its parents, being plain greenish or olive above and pale 

 underneath. It is comparatively unaffected by environment, though examples 

 living among green vegetation or on a dark background are somewhat darker than 

 those from a light bottom. The mosquito fish, unlike many other fresh and brackish 

 water species, does not acquire bright hues during the breeding season. 



The most brilliantly colored fishes, as a rule, occur on tropical coral reefs. Some 

 of the reef fishes, as, for example, the angelfish, butterflyfish, and labroid and 

 parrotfish, rival butterflies and moths in the display of bright colors and in their 

 many hued patterns. Many fish undoubtedly bear protective colors, that is, colors 

 that blend with the background and accordingly make them inconspicuous. It is 

 difiBcult to imagine that the bright colors of the coral-reef fishes could serve such 

 a purpose, yet those who have made observations under water have stated that 



